Experience Shows Wet Compression Is Safe to Use on Gas Turbines

Back in the early 2000s, some controversy emerged about the practice of spraying large amounts of fog directly into the compressor. Some claimed that this led to blade erosion, pitting, and scaling. So, is it safe to use wet compression on gas turbines by pumping water droplets into the compressor inlet? The answer is yes—provided the system produces small droplets, is from a reputable supplier, that demineralized water is employed, and that it is operated and maintained correctly.

What Is Wet Compression?

Wet compression or overspray consists of injecting demineralized water in the form of tiny fog droplets into the inlet of a gas turbine to improve output and heat rate (Figure 1). These droplets evaporate inside the compressor, which lowers the temperature and the work of compression. As a result, more power is available at the output shaft. Estimates of its effectiveness vary from 5% to 10% power increase for each 1% of water injected into the compressor inlet air mass flow. Some wet compression adopters have installed systems that spray as much as 2.5% of the air mass flow.

There have been many firm advocates of wet compression over the years. Steve Ingistov, principal engineer at Tesoro Corp.’s Watson Cogeneration Plant in Carson, California, has been using it successfully since the 1990s. The plant has four natural gas–fired GE 7EA turbines rated at 83.5 MW each (ISO), as well as two Siemens Energy steam turbines and a duct-fired heat recovery steam generator (HRSG) for a total of 385 MW. Ingistov is known for introducing a series of improvements, innovations, and upgrades to the facility that boosted output to 425 MW. Fogging and wet compression account for about 10 MW of that gain, he estimated.

First-row blade erosion was discovered at Watson Cogen in 2000. A detailed investigation traced it to the online water washing system and an aging evaporative cooler that was releasing very large droplets into the airflow. These large droplets agglomerated and were carried into the compressor.

The evaporative cooling system has since been replaced with fogging. Compressor blades were upgraded to Carpenter 450 at that time to avoid erosion due to water washing. Since then, Watson Cogen has accumulated two decades of liberal usage of wet compression without issue.

“During more than twenty years of operation, the MeeFog wet compression system has not caused blade erosion,” said Ingistov. “We run the system around the clock during the peak period of electricity generation from June to September and anytime the ambient temperature is above 50F.”

Supportive Wet Compression Users

Many other plants have deployed wet compression without issue, both in North America and internationally. The bulk of these were designed and installed by Mee Industries Inc. Another example is Eagle Point Power Generation (EPPG) in Westville, New Jersey. This 225-MW combined cycle facility consists of two GE Frame 7EA turbine generators, two Nooter/Eriksen HRSGs, and a single 50-MW Alstom condensing steam turbine. The facility began operating in 1991.

It had been using inlet air fogging for evaporative cooling for many years before a Rockland Capital affiliate purchased the generating facility in April 2012. That signaled a whole range of steps to boost the output, including upgrading the original inlet fogging system and adding wet compression.

“Depending on ambient conditions, EPPG will get an additional 12 MW total from the GE 7EAs, and there may also be an additional MW from the steam turbine,” said Jeff Zelik, EPPG’s plant manager. “This increase is from both the inlet fogging and the wet compression. The compressor inlet temperature will drop more than 20F in warm weather. Borescope inspections show that wet compression keeps the compressor clean so intervals between water washes can be extended.”

He added that the MeeFog system has performed well with no erosion, but cautioned others about the importance of keeping the filters clean and ensuring there is a good fogging pattern. With those points taken care of, running both inlet fogging and overspray is a reliable and safe way to get more value out of existing equipment, he added.

Like Ingistov, EPPG’s Plant Project Manager Dan Turley found online water washing to be the culprit for blade erosion found decades ago on its two GE Frame 7FAs. “Fogging is an integral part of our plant economics,” said Turley. “Having a properly designed and functioning fogging system on both units has made us a tremendous amount of money compared to the cost of the system.”

Another Recent Wet Compression Example

The Mesquite Block 2 plant, owned by Onward Energy, implemented wet compression about three years ago (Figure 2). The main goal was to increase power output throughout the year, especially during the Arizona summer when daily highs can average greater than 100F for four straight months. Block 2 consists of two GE 7FA.03 gas turbines, a D11 steam turbine, and Nooter/Eriksen HRSGs. The gas turbine’s maximum rating is 180 MW and the steam turbine provides 321 MW. Block 2 operates in baseload most of the year.

Onward installed a MeeFog wet compression system in each turbine in the spring of 2021, just in time for the hot summer months. Each unit can spray up to 35 gallons per minute with 90% of the flow being in droplets of 20 microns or less. The spray nozzle arrays are located in the vertical section of the inlet, downstream of evaporative coolers that continue to operate. The wet compression system uses a single high-pressure pump with a variable frequency drive to provide two stages of demineralized water spray.

After running the system for three years, Todd Hart, director of Gas Engineering at Onward Energy, gave the blades a clean bill of health. This is based on inspection of its blades every summer to ensure they remain in tiptop condition. The most recent inspection included digital R0 scans (the forward rotor blade in a gas turbine’s compressor) and nondestructive examination (NDE) testing.

“Our wet compression system produces 7 MW per 7F.03 unit within two to three minutes. We have not had any issues with erosion or scaling,” said Hart.

Good Design Ensures Wet Compression Success

Mee Industries is the foremost proponent of wet compression. It has installed fog systems in more than 1,000 turbines. About a third included wet compression capability. Over the last five years, the company has noticed an upsurge of customers requesting wet compression. They view it as an inexpensive alternative to major turbine upgrades or replacing older units with the latest generation of machines.

“As with any technology, inlet fogging must be done properly to produce the desired result,” said Thomas Mee III, CEO of Mee Industries. “We have more than 25 years of field experience with wet compression and our systems are designed to operate safely and effectively.”

Mee laid out the key factors that keep compressor blades free from erosion, corrosion, scaling, and pitting.

Droplet Size. Well-designed fogging nozzles produce billions of tiny droplets per second. This exposes more surface area of water to the air so they evaporate faster than larger droplets and provide more effective cooling. A given volume of spray consisting of 40-micron droplets exposes only one-fourth the surface area as compared to the same volume of spray consisting of 20-micron droplets.

“Research and experience show that inlet fogging droplets should be about 20 microns or less,” said Mee. “Our impaction pin nozzles provided droplets with 90% of the flow being in droplets of 20 microns or less.”

The following chart (Figure 3) was developed based on research done at Mee Industries. It can be used to estimate the droplet size produced by a proposed wet compression system.

Demineralized Water. Untreated water contains minerals that can lead to blade fouling and hot-gas-path corrosion. Therefore, using Demineralized water is crucial for fogging and wet compression systems.

Good Drainage. Compressor blade erosion can occur if flowing or pooled water is sucked into the compressor inlet. This may cause erosion of compressor blades over extended periods of time. Therefore, it is important to utilize properly designed drainage systems to remove water from the inlet duct. Water diverters should be utilized to direct water to drainage points. Additionally, a false floor above drain points can allow water to flow to the drain and not be drawn into the compressor.

Droplet Distribution. Inlet fogging systems must evenly distribute droplets in the airflow. This is best achieved with a larger quantity of fog nozzles, each with a lower flowrate, rather than fewer, high-flowrate nozzles. Such a system will also have nominal accumulation of flowing or pooling water in the inlet ducts, and at a given operating pressure, low-flow nozzles generally produce smaller droplets than high-flow nozzles.

Dirt and Rust. Inlet surfaces, including the compressor inlet, should be thoroughly cleaned before operating a fogging system. Dirt that has accumulated on duct surfaces or the compressor inlet can be carried to the compressor and cause fouling. Areas of rust or compromised coating should also be cleaned and recoated before operating a wet compression fogging system.

“While some early wet compression systems from inexperienced vendors may have contributed to blade erosion, MeeFog systems have recorded hundreds of thousands of hours of problem-free wet compression operation over more than a quarter of a century,” said Mee.

—Drew Robb ([email protected]) has been a full-time freelance writer for more than 25 years specializing in engineering and technology.